Dental materials which are used e.g. as cement or as direct filling material generally contain a polymerizable organic matrix and one or more fillers, which are usually surface-modified with a polymerizable adhesion promoter. The filler content depends decisively on the intended use and can be up to 90 wt.-%, wherein fixing cements have a lower level of filling compared with filling materials. The polymerizable organic matrix usually contains a mixture of resin monomers, initiator components, stabilizers and pigments. Dental materials which contain a polymerizable matrix and filler are called composites. The polymerizable matrix is also called resin.
Mixtures of dimethacrylates are usually used as monomers. Widely used examples of these are the highly viscous dimethacrylates 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)-phenyl]propane (bis-GMA) and 1,6-bis-[2-methacryloyloxy-ethoxycarbonylamino]-2,2,4-trimethylhexane (UDMA) and the dimethacrylates of lower viscosity, used as diluting monomers, bis-(3-methacryloyloxymethyl)tricycle[5.2.1.02,6]decane (TCP), decanediol-1,10-dimethacrylate (D3MA) and triethylene glycol dimethacrylate (TEGDMA). During polymerization, dimethacrylates bring about a three-dimensional cross-linking of the polymer chains that form and thus result in an improved mechanical stability.
The materials usually also contain an initiator for the radical polymerization, wherein light-curing materials which contain a photoinitiator occupy a dominant position in dental filling therapy today.
A disadvantage of light-curing materials is that in particular the fitting of larger fillings is associated with a considerable outlay, because the light required for curing can only penetrate into the materials up to a limited depth. In the so-called incremental technique, the filling is therefore built up of the composite material in layers, wherein the layers have a thickness of approx. 2 mm in each case and are cured individually.
Recently, so-called “bulk fill” composites, which allow layer thicknesses of from 4 to 5 mm, have attracted a great deal of interest due to the possible time saving. A large through-curing depth is a prerequisite for the clinical suitability of these materials. This correlates with, among other things, the translucence of the materials, wherein the translucence is decisively influenced by the refractive indices of the resin matrix and of the fillers. In composites, a high translucence and thus a good through-curing depth can be achieved when the organic matrix and the fillers used have matching refractive indices.
Bulk fill filling composites currently used are characterized by a high translucence before and after curing. This is an advantage with regard to the through-curing depth, but a disadvantage of this is that, due to the high translucence, the composites poorly mask the dentine lying below them, which is undesirable for aesthetic reasons because the colour of the dentine differs from that of the visible tooth enamel.
A further problem is the polymerization shrinkage stress (PCS) which builds up during polymerization and increases as the layer thickness increases. A further difficulty is that the PCS is particularly high during the light curing (Braga et al., Dent. Mater. 21 (2005) 962-970).